This invention relates in general to means for varying the speed of an AC (alternating current) motor, and in particular to means for varying the frequency and amplitude of the power supplied to the motor.
One way in which to vary the speed of an AC motor is to vary the frequency of the power being supplied. Normally the line power comes from a utility company at the standard 60 cycle per second frequency. There are circuits that will convert the standard frequency to different frequencies. These circuits also change the amplitude in proportion to the frequency change for efficient operation of the motor.
In commonly owned U.S. patent application Ser. No. 313,043, filed Oct. 19, 1981, Dick L. Knox, John M. Leuthen, and Kevin Lockyear (all of which is hereby incorporated by reference) a variable speed drive circuit is shown. In that circuit, a rectifier converts the AC three phase power supply into a DC (direct current). A FET (field effect transistor) switch is connected between the negative rail and one of the power conductors leading to the motor for each of the three phases. Similarly, three FET switches connect the positive rail to each of the three power conductors. These FET switches are switched on and off to produce a desired alternating current waveform of a selected frequency. Means are also employed to vary the amplitude in proportion to the frequency selected.
To accomplish these functions, a variable frequency oscillator is employed to provide pulses of frequency depending on the input selected. A binary counter counts these pulses up to a certain number, then repeats. For each count, the counter provides a binary output. A ROM (read only memory) receives the binary outputs and provides a programmed output for each of the FET switches to control the switching as determined by the frequency of the oscillator. The oscillator is controlled by a variable DC demand voltage.
Improvements to the system shown in the above-identified application are desirable. If excessive heat builts up within the variable speed drive system, no provisions are shown in the above-identified application to handle this occurrence. In the past, heat sensors normally completely cut off the motor if a maximum temperature is exceeded. This requires restarting of the motor and results in down time.